ESD protection mechanisms generally work in two ways. First, by dissipating the ESD current transient safely using a low-impedance discharging channel that prevents thermal damage in the structures of the integrated circuit. Secondly, by clamping any ESD induced voltage to a safe level to avoid dielectric degradation or rupture. Ideally the complete ESD protection solution should be realized on the integrated circuit (IC) creating an effective discharging channel from any pin to every other pin on the integrated circuit.
Devices that are used as ESD protection elements include diodes, bipolar transistors, metal-oxide-semiconductor field effect transistors (MOSFETs), and silicon-controlled rectifiers (SCRs). SCRs function as switches that can be configured to turn on and shunt voltage from the input/output (I/O) pads of an integrated circuit to ground.
In ESD protection some integrated circuit elements may be vulnerable by discharges occurring within automated equipment, while others may be more prone to damage from handling by personnel. This can occur from direct transfer of electrostatic charge from the human body or from a charged material to the electrostatic discharge sensitive (ESDS) element. When one walks across a floor, an electrostatic charge accumulates on the body. Simple contact of a finger to the leads of an ESDS device or assembly allows the body to discharge, possibly causing device damage. The model used to simulate this event is the Human Body Model (HBM).
The HBM testing model represents the discharge from the fingertip of a standing individual delivered to the device. It is modeled by a 100 pF capacitor discharged through a switching component and a 1.5 kOhm series resistor into the component. Typically, integrated circuit designers would like to see protection from the HBM testing to be greater than 2,000 volts.
An electrostatic discharge can also occur from a charged conductive object, such as a metallic tool or fixture. To test for this, designers use the Machine Model (MM). The machine model consists of a 200 pF capacitor discharged directly into a circuit without a series resistor. Typically, integrated circuit designers would like to see protection from the machine model to be greater than 200 volts.